The goal of this project is to understand how the endoplasmic reticulum (ER) of barrier epithelial cells lining the intestine may operate to innately sense the microbial environment, and how this may impact gut immunity, host defense, and the pathogenesis of Inflammatory Bowel Disease (IBD). Intestinal epithelial cells are highly susceptible to perturbations in the Unfolded Protein Response (UPR) and/or Endoplasmic Reticulum (ER) stress, a fundamental biological response originating from an alteration of processes normally occurring within the ER lumen. Genetic studies have revealed several ER stress/UPR- associated genes as risk factors for IBD, and they are associated with alterations in host-microbe interactions in the intestine and with the activation of key proinflammatory pathways. Remarkably, the intestinal epithelium of IBD patients often shows evidence of ER stress, implicating a paradigm of ER stress-related inflammation in IBD. We recently discovered the novel mechanism that IRE1a, the most ancient of the three mammalian ER stress sensors, acts as a pattern-recognition molecule in innate immunity to sense bacterial and viral proteins entering the ER from the lumenal environment (Cell Host Microbe, May 2013). Previously, there were no known mechanisms of innate immunity located in the ER (or Golgi) of host cells, though all other membrane- bound compartments and the cytosol are equipped with such factors. We believe the IRE1 pathway may act as a generalized mechanism of innate immune surveillance in the human intestine. Mammals, for example, have a second isoform of IRE1 (IRE1) whose expression is restricted to the heavily microbial colonized epithelium of the gut, and whose absence predisposes to colitis. We have preliminary evidence that IRE1 can act like IRE1a in pattern-recognition; and with enhanced activity in the innate immune signaling pathway. We hypothesize that IRE1, or both IRE1a and IRE1, may act to sense the gut microflora to set the immunologic tone (or response) of the intestinal mucosa in ways relevant to health and disease. In this grant, we will determine the mechanism and function of IRE1 as a pattern-recognition molecule in intestinal epithelial cells, and elucidate the physiologic consequence of the IRE1 signaling pathway for sensing the gut microflora and affecting mucosal immunity.